According to the state of the prior art, the oil supply of a change-under-load transmission provides, on one hand, for the lubrication and cooling of transmission components and, on the other hand, for their control by hydraulic means.
For the hydraulic transmission control of a change-under-load transmission, a continuous pump delivery flow is needed so that, at any desired point in time, the gear-shift elements can be filled without delay. During steady state operation, on the other hand, a continuous high pressure of about 15 to 30 bar is needed in order to hold the engaged gears or the variator discs steady, and a smaller delivery flow is required for this.
For a gear-shift element change, a change-under-load transmission at first needs a large delivery flow, at relatively low pressure, to fill the gear-shift element, and then, during modulation, when the clutch disc is engaged, a higher pressure with a low delivery flow, for example.
In addition most of the delivery flow is needed for lubrication and cooling of the transmission, and a low pressure of about 2 to 4 bar must be established for this.
A demand-orientated oil supply system of the prior art usually comprises two separate delivery flows with different pressures, which can be adjusted as necessary, as described for example in the context of patent specification EP 0 550 098 B1. This discloses an oil supply arrangement having a first and a second mechanically driven pump hydraulically coupled to one another in such manner that the control system of the second pump can be regulated, so that the demand-orientated oil supply can be adjusted by the pumps, by means of a regulator device.
This oil supply has the disadvantage that because of the separate delivery flows or oil circuits and the two mechanically driven pumps, its construction is very elaborate and entails high production and maintenance costs.
Further, in the context of DE 101 262 60 by the present applicant, it is proposed to provide the necessary, demand-orientated oil supply by means of a transmission oil pump, which delivers oil at high pressure only during a gear shift operation but which, during steady state, is operated at a low pressure level on the order of 3 to 5 bar.
To produce the required small delivery flow at high pressure at times other than during gear shift operations, as required in order to hold the engaged gears or the variator discs steady, a pressure-volume flow converter is provided, which is integrated into the oil circuit. For a low-pressure delivery flow, this pressure-volume flow converter produces a smaller delivery flow at high pressure, and corresponds to a motor/pump combination. High-pressure and low-pressure feed lines, respectively, are provided for the high- and low-pressure delivery flows.
This proposed solution also takes up structural space and involves several mechanical components.